President Donald Trump took aim at China on Thursday, saying that it hadn't followed through with vows to increase purchases of American agricultural products.

"China is letting us down in that they have not been buying the agricultural products from our great Farmers that they said they would," Trump wrote on Twitter. "Hopefully they will start soon!"

Trump said that China had agreed to buy an unspecified amount of American farm exports following his meeting with President Xi Jinping at the G20 summit in late June, when the two sides reached a ceasefire in their yearlong trade dispute. But China has not publicly acknowledged any such commitment.

"China is going to be buying a tremendous amount of food and agricultural product, and they're going to start that very soon, almost immediately," Trump said following his lunch with Xi. "We're going to give them lists of things that we'd like them to buy. Our farmers are going to be a tremendous beneficiary."

The New York Times reported Wednesday that Chinese officials denied that any concrete agreement on agricultural purchases had been reached last month, citing people familiar with the negotiations. The White House and the Office of the US Trade Representative did not respond to emails requesting comment.

Chinese purchases of key American agricultural products came to a standstill last year after Xi retaliated against the US with steep tariffs on soybeans and other grain products. The Trump administration has sought to placate farmers ahead of the 2020 elections, rolling out a multibillion dollar bailout package.

In May, nearly a dozen rounds of trade negotiations stalled after the US said China reneged on separate commitments. Talks resumed this week, but there was little evidence of progress on key issues such as intellectual property theft and enforcement. A US official said those meetings will continue "as appropriate," but did not offer further information.